Monday, March 27, 2017

Vintage charts, and the variation within a wine-growing region

I noted in a previous blog post that not everyone is enamored of vintage charts (Modern wine vintage charts: pro or con?). Such charts provide a quality score for each wine vintage in some specified wine-making region.

One of the objections to these charts is that the ratings over simplify — there is quality variation between vineyards even within local areas, and this is not taken into account. That is, not all wine producers will produce high-quality wine in allegedly good vintages; and not all wine producers will produce poor-quality wine in allegedly bad vintages.

I thought that it might be interesting to illustrate this point in practice.

I will do this using some data for the wines of Bordeaux, taken from Michel Dovaz' book: Encyclopedia of the Great Wines of Bordeaux (1981; Julliard).

For each classified wine-producing chateau, Dovaz provides a wine quality score for each vintage from 1970-1980 (plus some others). The scores indicate the quality of each vintage relative to the other vintages from the same producer, with the best one scaled as a score of 10. This means that the relative quality of each vintage can be directly compared between producers, without any concern about whether some producers are better than others — all producers have at least one vintage with a score of 10. [Technically, the data have been standardized to the same maximum value.]

We can look at these data for each of the different areas within the Bordeaux region, to see how much local variation there is in success for each vintage. Do all of the producers have success, or lack of success, in the same years? If so, then a vintage chart would be a very convenient way to tell us which were the successful years.

The first graph covers the red wines of the commune of Margaux, north of the city of Bordeaux. Each colored line represents the quality scores of a single chateau, with the solid line being the average of the scores in each year.

This graph alone represents the general point rather well. The chateaux do follow a single main pattern through time, but there is great variation around the average quality score. In particular, the chateaux do not all follow the same rank order of vintage quality. For example, 1970 was a better vintage than 1971 for most of the chateaux, but for one of them it was the other way around. Alternatively, 1976 was a better vintage than 1977 for all of the chateaux, although it was much better for many of them but only a little bit better for others.

The next graph covers the red wines of the commune of St Julien, which is the next one north of Margaux.

There are fewer chateaux represented here, but there is even more variation among these chateaux than there was for the Margaux area. Note, however, that there was very little variation in the 1975 vintage compared to the others.

The next graph covers the red wines of the commune of Pauillac, further north.

It is very similar to the previous graphs, being somewhat intermediate between the two of them.

There are too few classified chateaux in Saint-Estèphe to plot a worthwhile graph. So, we now move on to the area south of Bordeaux; and the next graph covers the red wines of the Graves region. There are too few Graves white wines to plot a worthwhile graph.

There are also few chateaux represented here. Note that for the first three vintages there is actually not much variation among the chateaux, but this changes for the rest of the decade, especially 1974.

The next graph covers the red wines of Saint Émilion, which is on the Right Bank of the Bordeaux region, rather than the Left. The adjacent area of Pomerol has data for only one chateau, and so it is included here, as well.

Clearly, the patterns of within-vintage variation are much the same for the Right Bank as for the Left. That is, not all of the chateaux follow the average pattern of variation, but most do, even though there is considerable variation among them.

Next, we can move on to Sauternes, the most famous white-wine region of Bordeaux, which makes very sweet wines.

This is the most expressive graph of all, regarding within-vintage variation. The making of high-quality sweet wines requires that the grapes be infected with Noble Rot (a fungus), and this process relies on very specific weather patterns. There were a number of years in the 1970s when the wine was deemed by a number of chateaux to be not worth releasing (scored 0 in the graph). Only in 1970 and 1975 did most of the chateaux produce high-quality wines.

Clearly, a vintage chart for Sauternes is not necessarily a reliable indicator of vintage quality. On the other hand, for the red wines shown in all of the previous graphs, such a chart would be useful in general. It would not, however, be a substitute for more detailed knowledge about each chateau.

Finally, lest we think that the results above are unique to the scoring system of Michel Dovaz, or are unique to the particular decade concerned, we could look at an independent source of data. The final graph shows the vintage quality scores from the Wine Spectator magazine, for the four "first growths" of the Médoc, covering the subsequent 33 years.

Even for these restricted chateaux, the within-vintage variation is large — the 1992 vintage was particularly variable. Note, however, that in this case the data have not been standardized, so that the within-year variation includes intrinsic variation in quality between the chateaux themselves (which may be small!).

About this blog

In the interests of doing something different to every other wine blogger, this blog will delve into the world of wine data, instead of wine itself. The intention is to ferret out some of the interesting stuff, and to bring it out into the light, for everyone to see. In particular, I will be drawing pictures of the data — as William Playfair said (in 1805): "whatever can be expressed in numbers may be represented by lines". Hopefully, this will be both interesting and informative.